The present invention relates generally to liquid crystalline light modulating devices, and more specifically to polymer stabilized bistable liquid crystalline devices which eliminate an undesired state in their displays.
Several applications of nematic liquid crystal devices have been reported that utilize bistability. This feature is important, since it provides a pronounced memory of the last imprinted ON-OFF state to the resulting display, in the absence of any field.
However, a characteristic of certain of these bistable devices is that a high pretilt angle is required in order to avoid stripe formation. As a result, the devices were constructed using evaporated SiO as the alignment layers. Such a procedure, however, is an elaborate and expensive process.
A more economical approach to liquid crystal device fabrication is the use of polyimide alignment layers. However, stripe formation in the resulting display is always a concern when using standard polyimide alignment layers. It would be desirable, therefore, to provide a liquid crystal device containing liquid crystal material exhibiting bistability which is capable of being economically fabricated, while avoiding stripe formation.
Another problem with prior art displays is the formation of undesired states for the liquid crystal. After removing a large field (approximately 40V), a type of liquid crystal display can relax into one of two desired states having a twist angle of xcfx86 and xcfx86+360xc2x0, wherein xcfx86 is in the range of 0xc2x0 to 180xc2x0. As an example, consider the case where xcfx86=0xc2x0. The backflow effect can be utilized to switch between the two states of 0xc2x0 and 360xc2x0 twist; however, the cell thickness to pitch ratio (d/P0) where switching occurs in this type of device (see FIG. 7) is also where the energy of an undesired intermediate twist state (180xc2x0 for the case considered here) is lower than that of either the 0xc2x0 or the 360xc2x0 states. This effect has required that after a short period of time (about 1-2 seconds) domains of the unwanted 180xc2x0 state will grow back and xe2x80x9cerasexe2x80x9d information present on the display. It is possible to refresh the display every 1 to 2 seconds to eliminate this problem; however, this technique substantially increases power consumption and the refreshing occurs at a rate which is distracting to a viewer.
Another technique used to overcome this 180xc2x0 twist state formation was the creation of a high pretilt angle in the spacing between the pixels. The high pretilt significantly increases the energy of the 180xc2x0 twist state thus inhibiting its formation. However, this method requires expensive and difficult manufacturing techniques.
It is an object of the present invention to provide a bistable liquid crystal device which substantially eliminates undesired states.
The present invention provides a liquid crystal display device including a polyimide alignment layer, having a bistable liquid crystal material and a polymer stabilizer in an amount from about 1% to about 15% based on the total weight of polymer and liquid crystal material, wherein the liquid crystal provides a twist angle from 0xc2x0 to 360xc2x0.